I'm the same as Joe. Men from my dad's family are all NW1s till death, but men in my mum's family are a mixed bag: There is a NW2 one at 44 and the rest end up NW5A-NW6 by their 40s-50s.
I've read some scientific papers that point that the overall hair sensitivity to DHT might be influenced by your maternal genes, and your overall DHT levels and 5-alpha-reductase activity (The enzyme that produces DHT) might be influenced by your paternal genes. So for example, you might inherit high DHT levels from your father, but low or no hair sensitivity to DHT. This might be the case of Arnold Schwarzegger, whose dad was bald by his mid/late thirties, yet Arnie never went bald despite blasting steroids at a time when little was known about them.
On the other hand, someone might have inherited low DHT levels from his dad, but might have moderate or high sensitivity to DHT. In this case the hair loss should be slower or milder. I've read from users here that the hair's sensitivity to DHT tends to increase as we age but I've no idea if it is true or not.